A tripod type constant velocity joint comprises an inner member having three trunnions, an outer member having three guide grooves, and three roller units respectively disposed rotatably between the trunnions and the guide grooves. These roller units are classified into what are called a single roller type and a double roller type. The single roller type roller unit comprises needle bearings disposed rotatably on an outer periphery of a trunnion, and an outer roller disposed on an outer periphery of the needle bearings so as to be capable of rolling in a guide groove (See Japanese Unexamined Patent Publication (KOKAI) No. 2000-220655, for instance) On the other hand, the double roller type roller unit comprises an inner roller disposed rotatably and swingably on an outer periphery of a trunnion, an outer roller disposed in a guide groove so as to be capable of rolling, and needle bearings disposed between the inner roller and the outer roller (See Japanese Patent No. 2753956, for instance). That is to say, the double roller type roller unit can swing with respect to the trunnion.
Snap rings are employed to secure the roller unit components in place. In the case of the single roller type roller unit, the snap rings are secured to the trunnion. This is because the roller unit moves only axially with respect to the trunnion and does not swing with respect to the trunnion. On the other hand, in the case of the double roller type roller unit, since the inner roller swings with respect to the trunnion, the snap rings cannot be secured to the trunnion. Therefore, in the case of the double roller type roller unit, the snap rings are secured to the outer roller. Namely, the outer roller has engaging grooves on its inner circumferential surface and the snap rings are fitted in these engaging grooves.
By the way, when a shaft to which a constant velocity joint is connected has a small diameter, there is a demand for downsizing the constant velocity joint. In the tripod type constant velocity joint employing the double roller type roller units, however, the snap rings to be fitted in the engaging grooves of the inner circumferential surface of the outer roller make it difficult to downsize the constant velocity joint. When the snap rings are fitted into the engaging grooves of the inner circumferential surface of the outer roller, first the diameters of the snap rings have to be reduced. As the snap rings have a smaller diameter, the ratio of a diameter shrinkage to the original diameter of the snap rings becomes larger. Therefore, there is a fear that stress placed on the snap rings might exceed yield stress and that the snap rings might be plastically deformed. As a result, the snap rings might be unable to return to their original diameters and engagement widths of the engaging grooves of the inner circumferential surface of the outer roller and the snap rings might become small. If thus the engagement widths become small, in some cases the snap rings may come off the outer roller.
Therefore, when there is a risk that the above problem will occur, it is conventional to use a constant velocity joint suitable to be connected to a larger-diameter shaft. For example, when a tripod type constant velocity joint employing the double roller type roller units is used for a vehicular power transmission shaft having an outer diameter of 19 mm or less, it is conventional to use a constant velocity joint suitable for a power transmission shaft having an outer diameter of more than 19 mm.